The objective is to build a high speed, high resolution laser scanning instrument to measure both the mass and the molecular weight of DNA separated by gel electrophoresis. The instrument detects visible fluorescence from a DNA-specific dye excited by a cw laser. The mass of DNA in a band is proportional to the summed fluorescent signal. A 100X improvement in detection limit over present photographic methods has been demonstrated together with the ability to simultaneously measure DNA fragments differing in mass by 104:1. The Phase II instrument is intended to scan a 20 cm sq. gel 5X to 8X faster than the PHase I breadboard. It will incorporate improved data handling techniques, means for further improving analytical performance, and will have the additional capability of performing as a transmission densitometer. An important goal is to limit the design approach to one which will lead to a moderately-priced commercial product. Molecular biology, genetic analysis, and DNA-based clinical diagnostics are among the areas which promise to benefit substantially from the improved performance. A number of researchers have confirmed that they perceive an opportunity for such improvement in their own work. They have agreed to provide gel samples for testing in an extensive evaluation program.